1. Field of the Invention
The present invention relates to a flat panel display device in which a pixel portion and a driving circuit for driving the pixel portion are integrally formed on the same substrate, and to a pumping circuit used for the flat panel display device.
2. Description of the Related Art
In recent years, an active matrix type liquid crystal display device in which a TFT (a thin film transistor) is disposed for each pixel as a switching element has come to a mainstream of a flat panel display device. Development of a liquid crystal display device has been lately progressed, in which a driving circuit and a power source circuit are integrally formed on an array substrate having a pixel portion formed thereon.
In the liquid crystal display device of such a kind, various kinds of clock signals and a power source voltage are supplied from an external control circuit to the driving circuit and the power source circuit on the array substrate. At present, in order to make the external control circuit in a compact size, it has been investigated to arrange a pumping circuit, which is included in the external control circuit, on the array substrate. However, since a capacitor having a large capacitance, which is necessary for the pumping circuit, cannot be formed on the array substrate in present fabrication processes, the external control circuit cannot be made in a more compact size. Furthermore, if the pumping circuit can be fabricated in the same fabrication processes as those for fabricating the pixel portion and the driving circuit, the pumping circuit can be fabricated in cost lower than a case where the pumping circuit is mounted on the external control circuit. Nevertheless, as described above, since the pumping circuit cannot be formed on the array substrate, it is impossible to fabricate the pumping circuit in low cost.
On the other hand, an operation to form a TFT on a glass plate like an array substrate makes fabrication processes difficult, and transistor characteristic are made to be unstable in many cases. Accordingly, when the pumping circuit is formed on the array substrate, the characteristics of the TFT constituting the circuit are expected to become unstable. In this case, a problem that variations in a threshold occur and an output voltage from the pumping circuit becomes unstable is caused.
Furthermore, when the TFT on the array substrate is formed of polycrystalline Si, performance of the TFT formed of the polycrystalline Si is inferior to that formed of a single crystal Si. When a gate length of the TFT formed of the polycrystalline Si is made to be longer to compensate the inferiority to the TFT formed of the single crystal Si, an area required to a circuit arrangement becomes large. Accordingly, in the case where the pumping circuit arranged on the array substrate is constituted of the TFT, a problem that a fringe region around the pixel portion becomes large is caused.